A centrifugal blower unit of certain kind hitherto known comprises a scroll casing provided with an outlet port of a rectangular shape in cross section, and uses a flared air passage communicating between the outlet port and an air duct, the flared air passage having a rectangular shape in cross section with a height expanding continuously from one end connecting the outlet port toward the other end connecting the air duct (Patent literature 1, for example).
Referring now to FIG. 9 and FIG. 10, description is provided hereinafter of such centrifugal blower unit. FIG. 9 is a side view showing a structure of a conventional centrifugal blower unit, and FIG. 10 is a plan view of the same centrifugal blower unit.
As shown in FIG. 9 and FIG. 10, scroll casing 105 is provided with rectangular outlet port 101 having a height dimension H1 formed between anti-tongue-side wall 103 facing tongue-like portion 102 and tongue-side wall 104, and side casing plate 107 having inlet port 106 on each side of scroll casing 105. Centrifugal blower unit 109 is also provided with impeller 108 disposed inside scroll casing 105. Air duct 110 of a rectangular shape is formed into height H2, which is larger than height H1. Rectangular flared air passage 112 is formed between outlet port 101 and air duct 110 with wall surfaces 111 having length L. Wall surface 111 is sloped toward tongue-side wall 104 at an angle to a plane extended from anti-tongue-side wall 103 in a manner that a height of flared air passage 112 expands continuously from one end connecting outlet port 101 of height H1 toward the other end connecting air duct 110 of height H. Rectangular flared air passage 112 allows air to flow steadily in main-flow area 114 since one of wall surfaces 111 sloped at angle T to the plane extended from anti-tongue-side wall 103 toward tongue-side wall 104 is connected continuously to air duct 110 to reduce backflow area 113 inside air duct 110.
The conventional centrifugal blower unit of the above type can produce a steady airflow inside main-flow area 114 when width W1 of rectangular outlet port 101 and width W2 of rectangular air duct 110 are dimensionally equal. If width W2 is different from width W1, however, and leaves a difference in co-planarity from outlet port 101 to air duct 110 as shown in FIG. 10, it results in a loss of pressure and produces turbulent noise attributable to turbulent airflow 115 due to sudden expansion and collision of the flowing air, thereby decreasing a static pressure and increasing an input power and noise of centrifugal blower unit 109.
It is also known that centrifugal blower unit 109 generally produces side airflow 116 of a low velocity near the end plates of the casing, which tends to cause the direction of airflow unstable and produce turbulent airflow. The turbulent airflow induces back flow 117 at the edge of impeller 108 toward a space between impeller 108 and inlet port 106 near tongue-like portion 102, which also causes a loss of pressure and turbulent noise, thereby giving rise to problems of decreasing the static pressure and increasing the input power and the noise of centrifugal blower unit 109.    Patent Literature 1: Japanese Patent Unexamined Publication, No. 1998-299697